Global feature flags via environment variables

[3mcd]

Issue(s) Resolved

Partial #1140

High-level Explanation of PR

This PR introduces a simple pattern for implementing global (i.e. not community-level) feature flags.

One would create a new feature flag by adding a tuple to the new flagSchema zod type in env.ts, where the first element of the tuple is a unique flag name, and the second element is the flag "args" or options. Default flag states/args are defined using Zod's .default method.

Turning a flag on or off is as simple as starting the app with FLAGS=flag-name:off. You can toggle multiple flags using a comma separated list, e.g. FLAGS=flag-1:off,flag-2:on.

Some flags may take slightly more complex options than just "off" or "on". For example, the disabled-actions flag has a transformer that parses the following flag expression:

disabled-actions:http+email

into

["http", "email"]

The state and/or arguments of a flag can be retrieved on the server like

env.FLAGS.get("disabled-actions") // ["http", "email"]
env.FLAGS.get("uploads") // false

The flags I've introduced here don't do anything yet. I intend to utilize them in the em/disabled-actions branch, and a small follow-up branch to disable file uploads and member invites in our sandbox environment.

Test Plan

1. Start the app with FLAGS=uploads:off,invites:off. It should start successfully! (These flags don't do anything yet).
2. Try running the app with an unsupported flag turned on or off. The app should fail to start with an environment error.
3. Try running the app with an unsupported flag state, e.g. FLAGS=uploads:pancakes. The app should fail to start again.

Screenshots (if applicable)

Notes

Setting a feature flag currently doesn't have any effect, this PR just introduces the pattern.

[3mcd]

All of the important changes are in this file!

[allisonking]

worked as promised! although just a note that the check does not kick in on server reload. if I make my .env.local invalid, I'll get a message on the server that says "Reload env: .env.local" but the server will keep working fine. I think this is okay, but I guess means we can't dynamically swap out feature flags without restarting the server?

[3mcd]

@allisonking I'm not totally sure what's causing the issue with reloading. Because we'll probably have a more robust solution in the future (i.e. one that supports per-community settings), I think I'm okay with the limitation of a server restart for global feature flags for now!